To insert a dynamic piece of text to a word document (multiple times, where required), you should use Custom Properties.
[Written for Word 2013, but relevant in 2010]
To do this, first open the document in question. Within the File menu, click the Info tab, select the Properties dropdown, and open Advanced Properties.
In the window that opens (see image, below), switch to the Custom tab, and enter appropriate values for Name and Value (Type should obviously be Text). Click Add.
You can then add this property to the document in one of a few ways: the easiest (arguably) is to go to the Insert ribbon, and select QuickParts (as in @Nichlas H.'s answer), and open the Field screen.
After navigating to the DocProperty field name (see above), you can select any property to insert into the document. This adds that property to the document, where it can be updated in a variety of ways.
Alternatively, you can hit Ctrl+F9 to insert an empty Field, into which you can type
DOCPROPERTY Test (or any other field code - see above).
You can update these fields by highlighting them (or everything, with Ctrl+A) and hitting F9, or right-clicking any field and selecting Update Field.